1. Field of the Invention
The present invention relates to apparatus for measuring the level of liquid, and more particularly to an apparatus wherein a liquid level is indicated by the position of a float relative to a scale, determined by transmission and detection of a radiated signal through the scale.
2. Brief Description of the Prior Art
There are a number of sensors and sensor systems to measure liquid levels. There is currently a need for lower cost sensors and sensor systems, particularly for the storage tank marketplace, that can be used for inventory management without sacrificing accuracy, reliability or functionality. The most common types of level measurement systems use floats.
In one type of system, the float is suspended by a cable from a circular storage reel located at the top of a liquid container. An optical reader is used to measure how much cable is rolled in or out from the reel. Such a system is described in U.S. Pat. No. 3,558,861 by Collins. This type of system has a number of mechanical drawbacks that affect both accuracy and reliability. One such drawback is the need for a motor and tensioning system to ensure that the tape not interfere with the movement of the float and yet accurately reflect the distance from the float (liquid level) to the optical reader.
An improved system is described in U.S. Pat. Nos. 4,014,015 and 4,079,627 by Gundlach. This system uses a rigid vertical member with a float allowed to freely move up and down with the level of the liquid. The vertical member has a number of electrical sensors, each representing a discrete location along the vertical member. The sensors interact magnetically with the float to produce a signal in the sensor indicative of float position. This system is structurally complicated because of the large number of sensors required for implementation.
Another apparatus using either optics or magnetics to determine the float position relative to a vertical member is described in U.S. Pat. No. 5,483,831 by Steiner. The magnetic implementation includes a series of horizontal binary coded bits for each linear division along the member length. Each division has magnetic and non-magnetic areas that are read by a horizontal array of magnetic reading sensors mounted on one side of the float. The optical implementation uses a series of horizontal binary coded bits for each linear division, each horizontal series having reflecting and non-reflecting areas that are read by an array of photo emitters and detectors mounted on the same side of the float. Each coded bit has a corresponding emitter and detector, and the distance between them and the reflecting element must be tightly controlled for accurate liquid level measurement. In order to accomplish this control, a special guide is required to align the float with the vertical member. The guide assures that the float reading head is at a fixed distance from the horizontal binary coded scale. One disadvantage of the Steiner device is that it is mechanically complex. Another disadvantage of the Steiner device is the possibility of contamination of the code when in operational use, which affects the reflections and degrades performance. Contaminants can also cause friction in the close fitting guide system, impeding the motion of the float and causing reading errors. In addition, since the optics of Steiner are located within the liquid, any liquid-surface anomalies can also cause refraction errors.
There are a large number of applications for the present invention, particularly those applications in which a simple, accurate, reliable and inexpensive sensor system is required to measure liquid level. The present invention can be used for measuring the level of any kind of liquid in any kind of manmade or natural containment system, including, for example, underground and aboveground fuel and chemical storage tanks, water tanks and various types of manmade and natural water containment systems, groundwater monitoring wells, and liquid-waste tank systems. This invention can also be used in manufacturing or processing systems and various types of instrumentation and measurement systems requiring liquid-level measurements. The present invention can be used in both pressurized and unpressurized containment systems and in both open and enclosed containment systems. While the main purpose of the present invention is to measure the level of a liquid over a range of depths, the same invention has applications as a level switch, which indicates that the level has reached a certain depth, and in applications where the change in level is required (e.g., leak detection applications). Also, the present invention can also be used to measure the level of the interface between two immiscible liquids.
The present invention is motivated by and fulfills the current market need for an inexpensive and functionally robust apparatus that has the accuracy to measure the level of liquid in underground storage tanks containing petroleum fuels and hazardous substances that will satisfy (1) the operational and regulatory requirements for inventory management, control and reconciliation and (2) the regulatory requirements for leak detection. While motivated by this application, some of the other applications are equally important and require the same type and accuracy of level measurements.
The regulatory requirements for inventory measurements specify that liquid level should be measured daily with an accuracy of 1/8 in., or better; this level of accuracy also addresses satisfactorily those inventory measurements used only for operational considerations and for leak detection using statistical inventory reconciliation (SIR) methods. SIR methods have been developed recently that can accurately detect small leaks in underground storage tanks using level or inventory data collected over a long period of time (e.g., a month). This method of leak detection can be implemented using the stick measurements typically used for inventory measurements in underground storage tanks. However, inexpensive application of this leak detection method requires "accurate and reliable" inventory level measurements, which are difficult to obtain using stick measurements. Stick measurements are difficult to use because they are typically filled with operator reading and recording errors. Automatic tank gauges (ATGs) can be used for making the level measurement required for inventory control and for SIR. However, ATGs are more expensive than required, because these systems also have the high level of precision required to measure very small level changes (e.g., typically 0.001 in.) and a means for thermal compensation of the liquid that is needed for conducting a leak detection test over a short time period (e.g., typically several hours to a day).